1. Field of the Invention
The present invention relates to a thermal displacement correction apparatus for a machine tool, and particularly, relates to a thermal displacement correction apparatus for a machine tool which automatically determines necessity/unnecessity of actual measurement of thermal displacement at each portion of the machine tool.
2. Description of the Related Art
A machine tool has a correction function with which a thermal displacement amount is calculated and corrected from an operation thereof, temperatures of portions thereof, and the like. In the correction function, there can be a case where the thermal displacement amount cannot be correctly calculated, which causes a large difference (correction error) between a thermal displacement correction amount for correcting the calculated thermal displacement amount and an actual thermal displacement amount and leads to incorrect correction. In such a case, an actual displacement amount of each portion of the machine tool is typically measured with use of a measurement device and is compared with the thermal displacement correction amount. With this comparison information, a calculation unit of the thermal displacement correction amount is controlled so as to reduce the correction error to modify the thermal displacement correction amount, and thereby, correction precision of the thermal displacement is improved.
As an example of an adjustment method of the thermal displacement correction amount in the conventional art, Japanese Patent Laid-Open No. 11-90779 discloses a method of modifying and adjusting the thermal displacement correction amount by using a measurement device in processing, measuring an actual displacement amount of each portion of the machine tool, and adding or subtracting, to/from the calculated thermal displacement correction amount, an adjustment value obtained by the comparison with the thermal displacement correction amount.
Moreover, when the actual displacement amount is measured with use of a measurement device, it is needed for measurement time (actual measurement timing) to be configured beforehand. Japanese Patent Laid-Open No. 11-114775 discloses a method of automatically determining necessity/unnecessity of actual measurement using information of a processing program, previous measurement time and the like. Moreover, the adjustment value is calculated and automatically changed by performing the actual measurement.
However, depending on a machine property and a processing environment, in the method of performing measurement at the preset measurement time and the method of performing measurement based on information of a processing program, previous measurement time and the like, there can be a case where the correction error cannot be grasped based on the adjustment value and necessity/unnecessity of actual measurement of thermal displacement at each portion of the machine tool cannot be properly determined. Therefore, in the conventional method, it is needed for the measurement time to be predetermined for measuring the actual thermal displacement, and the measurement time is needed to be changed for each processing program. Moreover, depending on the machine property and the processing environment in processing, there can be a case where necessity/unnecessity of the measurement cannot be properly determined.